31.

In the inductive circuit given in the figure, the current rises after the switch is closed. At instant when the current is 15  mA, then potential difference across the inductor will be

         

• zero

• 240 V

• 180 V

• 60 V

A wire of radius r has resistance R. If it is streched to a radius of $\frac{3\mathrm{r}}{4}$. Its resistance becomes

• $\frac{256 \mathrm{R}}{81}$

• $\frac{81 \mathrm{R}}{256}$

• $\frac{16 \mathrm{R}}{9}$

• $\frac{9 \mathrm{R}}{16}$

What is the area of the plate of a 3 F parallel plate capacitor, if the separation between the plates is 5 mm?

• 12.981 x 10² m²

• 9.281 × 10⁹ m²

• 1.69 × 10¹² m²

• 4.529 × 10⁹ m²

A magnetising field of 1600 A/m produces a magnetic flux 2.4 x10^-5 Wb in an iron bar of cross-sectional area 0.2 cm². The susceptibility of an iron bar will be

• 1788

• 1192

• 596

• 298

If the earth field induction at a place is 0.36 gauss and the angle of dip is 60°, the horizontal and vertical components of the field will be respectively

• 0.18 gauss, 0.18 $\sqrt{3}$ gauss

• 0.98 gauss, 0.9 $\sqrt{2}$ gauss

• 1.08 gauss, and 1.08 $\sqrt{3}$ gauss

• 1.07 gauss, 0.11 $\sqrt{2}$ gauss

The sun delivers 10⁴ W/m² of electromagnetic flux to the earth's surface. The total power that is incident on a roof of dimension (10 x 10) m²  will be

• 10⁴ W

• 10⁵ W

• 10⁶ W

• 10⁷ W

X-rays are not used for radar purposes because they are not

• reflected by target

• partly absorbed by target

• electromagnetic waves

• completely absorbed by target

An 8 µF capacitor is connected across 200 V, 50 Hz line.  What is the peak value of the charge through capacitor?

• 2.5 x 10^-3 C

• 2.5 x 10^-4 C

• 5 x 10^-5 C

• 7.5 x 10^-2 C

A wire carrying current i and other carrying is in the same direction produce a magnetic  field B at the midpoint. What will be the field when 2i current is switched off ?

• B/2

• 2B

• B

• 4B

A drift velocity offree electrons in a conductor is v_d, when the current i is flowing in it. If both the radius and current are doubled, the drift velocity will be

• $\frac{{\mathrm{V}}_{\mathrm{d}}}{8}$

• $\frac{{\mathrm{V}}_{\mathrm{d}}}{4}$

• $\frac{{\mathrm{V}}_{\mathrm{d}}}{2}$

• V_d